Parallel nested transactions in transactional memory

ABSTRACT

Various technologies and techniques are disclosed for supporting parallel nested transactions in a transactional memory system. Multiple closed nested transactions are created for a single parent transaction, and the closed nested transactions are executed concurrently as parallel nested transactions. Various techniques are used to ensure effects of the parallel nested transactions are hidden from other transactions outside the parent transaction until the parent transaction commits. For example, versioned write locks are used with parallel nested transactions. When a transactional memory word changes from a write lock to a versioned write lock, an entry is made in a global versioned write lock map to store a pointer to a write log entry that the versioned write lock replaced. When the versioned write lock is encountered during transaction processing, the global versioned write lock map is consulted to translate the versioned write lock to the pointer to the write log entry.

BACKGROUND

Software transactional memory (STM) is a concurrency control mechanismanalogous to database transactions for controlling access to sharedmemory in concurrent computing. A transaction in the context oftransactional memory is a piece of code that executes a series of readsand writes to shared memory. In other words, a transaction accesses datain one or more objects. An object in the context of transactional memoryis a set of connected memory locations that are locked as one entity. Anobject in this context might also be a static variable, or a set of suchvariables, or it might be a set of cache lines.

STM is used as an alternative to traditional locking mechanisms. STMallows concurrent programs to be written more simply. A transactionspecifies a sequence of code that is supposed to execute as if it wereexecuting in isolation. This illusion of isolation is achieved byfine-grained locking of objects, and by executing in a mode that allowsthe side-effects of the transaction to be rolled back if the transactionis discovered to be in conflict with some other transaction. We say thata data access is “transacted” if the code generated for that access hasbeen modified to include support for these locking and rollbackmechanisms.

Transactions may be nested, and can be classified as open or closednested. If a thread is currently executing a transaction and reaches thestart of a new atomic block, this atomic block is executed as a closednested child transaction of the currently-executing parent. This nestedtransaction executes within the same isolation boundary as the enclosingtransaction, and just like other memory accessed within the enclosingtransaction, the effects of the nested transaction will only becomevisible when the enclosing transaction commits. In other words, theparent transaction is effectively suspended, and the closed nestedtransaction is allowed to run to completion before processing in theparent is resumed. When a nested transaction rolls back, its temporaryeffects are undone and the state of the parent transaction is restoredto the point that the nested transaction began.

The “outermost” transaction being executed by a given thread is notnested; we call this the top-level transaction. This top-leveltransaction must execute atomically, so the nested transactions becomepart of it. Nesting could arise, for example, if some abstractions A andB each had internal representation invariants they wanted to maintaineven in use by concurrent threads, and they therefore used atomic blocksin the implementations of their methods to guarantee that theseinvariants are not violated by concurrent accesses. Now assume that somehigher-level abstraction C uses instances of A and B in itsimplementation, and has some invariant that relates these A and Binstances. Methods of C might use transactions to ensure that thisinvariant is not violated. If A and B methods are used inside C'stransactions, the transactions in the A and B methods will be nested (inthis use).

Current transactional memory systems do not allow work performed withinthe isolation boundary of one transaction to be distributed betweenmultiple concurrent threads of execution. In current systems, atransaction may have only one nested child transaction. The semantics ofsuch systems simply do not allow such parallelism within a transaction,and attempts to execute more than one nested transaction at a time wouldresult in nested transaction log entries intermixed without order in theparent's log and other errors, and a breakdown of the basic underlyingfine grained locking protocols used to provide the illusion ofisolation.

SUMMARY

Various technologies and techniques are disclosed for supportingparallel nested transactions in a transactional memory system. Multipleclosed nested transactions are created for a single parent transaction,and the closed nested transactions are executed concurrently as parallelnested transactions. Various techniques are used to ensure effects ofthe parallel nested transactions are hidden from other transactionsoutside the parent transaction until the parent transaction commits.

In one implementation, versioned write locks are used with parallelnested transactions. When a transactional memory word changes from awrite lock to a versioned write lock, an entry is made in a globalversioned write lock map to store a pointer to a write log entry thatthe versioned write lock replaced. When the versioned write lock isencountered during transaction processing, the global versioned writelock map is consulted to translate the versioned write lock to thepointer to the write log entry.

In another implementation, the release of a duplicate write lock forrollback is supported for parallel transactions. During rollbackprocessing of a parallel nested transaction, a first write log entry isencountered that represents a write lock. If the write lock isdetermined to be a duplicate, a global lock is acquired and used tosynchronize access to a global versioned write lock map.

In yet another implementation, optimistic read validation is supportedfor parallel nested transactions. During optimistic read validation, ifa versioned write lock indicates a conflict from a sibling parallelnested transaction, then information is consulted to determine if aparallel nested transaction should be doomed. In one implementation,that information is contained in a versioned write lock and in a globalversioned write lock map.

In yet a further implementation, write lock acquisition is supported forparallel nested transactions. Upon attempting to acquire a write lockfor a parallel nested transaction, a transactional memory word is readand analyzed to determined if the write lock can be obtained. If thetransactional memory word indicates a versioned write lock, a globalversioned write lock map is accessed to retrieve a write log entrypointer that points to a first write log entry.

In yet another implementation, pessimistic reads are supported forparallel nested transactions. A pessimistic duplication detection datastructure is created for a parallel nested transaction. An entry is madeinto the data structure for each pessimistic read in the parallel nestedtransaction. When committing the parallel nested transaction, newpessimistic read locks are passed to an immediate parent, and an entryis made into a separate pessimistic duplication detection data structureof the immediate parent with synchronization between siblingtransactions. The pessimistic duplication detection data structures canalso be used for upgrades from pessimistic reads to write locks.

In another implementation, retry operations are supported with parallelnested transactions. When a transaction that is a parallel nestedtransaction or a child transaction of the parallel nested transactionexecutes a retry, a read set of the transaction is registered for theretry. When a decision is made to propagate the retry past a parallelnested transaction parent of the transaction, the read set is keptregistered and made part of a parent read set.

In yet a further implementation, write abort compensation maps can beused with parallel nested transactions to detect and handle falselydoomed parent transactions. A write abort compensation map is createdwhen releasing new write locks for a parallel nested transaction duringrollback. When the parallel nested transaction rolls back, an entry iscreated in the write abort compensation map for each new write lockreleased.

This Summary was provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a computer system of oneimplementation.

FIG. 2 is a diagrammatic view of a transactional memory application ofone implementation operating on the computer system of FIG. 1.

FIG. 3 is a process flow diagram for one implementation of the system ofFIG. 1 that illustrates the stages involved in allowing multipleparallel transactions to be nested under a single parent transaction.

FIG. 4 is a process flow diagram for one implementation of the system ofFIG. 1 that illustrates the stages involved in allocating parallelnested transaction entries in the parent log when the number of parallelnested transactions is known up front.

FIG. 5 is a process flow diagram for one implementation of the system ofFIG. 1 that illustrates the stages involved in allocating parallelnested transaction entries in the parent log when the number of parallelnested transactions is not known up front.

FIG. 6 illustrates an example structure of a transactional memory wordof one implementation.

FIG. 7 is a process flow diagram for one implementation of the system ofFIG. 1 illustrating the stages involved in using versioned write locksto ensure parallel nested transactions nest and compose properly.

FIGS. 8 and 9 are process flow diagrams for one implementation of thesystem of FIG. 1 illustrating the stages involved in properly handling anested transaction acquiring a write lock.

FIG. 10 is a process flow diagram for one implementation of the systemof FIG. 1 that illustrates the stages involved in properly handling anested transaction releasing any kind of write lock for commit.

FIG. 11 is a process flow diagram for one implementation of the systemof FIG. 1 that illustrates the stages involved in properly handlingnested transaction releasing new write lock for rollback.

FIG. 12 is a process flow diagram for one implementation of the systemof FIG. 1 that illustrates the stages involved in properly handling anested transaction releasing a duplicate write lock for rollback.

FIG. 13 is a process flow diagram for one implementation of the systemof FIG. 1 that illustrates performing optimistic read lock acquisition.

FIGS. 14A-14D are process flow diagrams for one implementation of thesystem of FIG. 1 that illustrates performing optimistic read validation.

FIG. 15 is a process flow diagram for one implementation of the systemof FIG. 1 that illustrates the stages involved in ensuring correctoperations of pessimistic reads with parallel nested transactions.

FIG. 16 is a process flow diagram for one implementation of the systemof FIG. 1 that illustrates the stages involved in upgrading pessimisticread locks to write locks.

FIGS. 17A-17D are process flow diagrams for one implementation of thesystem of FIG. 1 that illustrates performing pessimistic read lockacquisition.

FIG. 18 is a process flow diagram for one implementation of the systemof FIG. 1 that illustrates the stages involved in allowing retry to workcorrectly with parallel nested transactions.

FIG. 19 is a process flow diagram for one implementation of the systemof FIG. 1 that illustrates the stages involved in using write abortcompensation maps with parallel nested transactions.

DETAILED DESCRIPTION

The technologies and techniques herein may be described in the generalcontext as a transactional memory system, but the system also servesother purposes in addition to these. In one implementation, one or moreof the techniques described herein can be implemented as features withina framework program such as MICROSOFT®.NET Framework, or from any othertype of program or service that provides platforms for developers todevelop software applications. In another implementation, one or more ofthe techniques described herein are implemented as features with otherapplications that deal with developing applications that execute inconcurrent environments.

As noted in the background section, a nested transaction is consideredclosed if it its effects are part of the same isolation boundary as itscontaining, or parent, transaction. Using various technologies andtechniques described herein, a transaction may have multiple closednested transactions at the same time. These are called “parallel nestedtransactions” (PNTs). All of the PNTs under a single enclosingtransaction are called the “parallel children” of the transaction, andthe enclosing transaction is called the “parallel parent”. The parallelparent and its children are referred to as a “parallel nest”. A“sibling” of a PNT is another PNT enclosed (at some nesting level)within the same parallel parent. In one implementation, each PNTexecutes much like a normal closed nested transaction: its effects areisolated within the enclosing transaction and only become visible beyondthe parallel nest when the parallel parent commits. However, each PNT isisolated from its siblings just as if it were a top-level transaction.The effects of a PNT only become visible to its siblings when itcommits.

As shown in FIG. 1, an exemplary computer system to use for implementingone or more parts of the system includes a computing device, such ascomputing device 100. In its most basic configuration, computing device100 typically includes at least one processing unit 102 and memory 104.Depending on the exact configuration and type of computing device,memory 104 may be volatile (such as RAM), non-volatile (such as ROM,flash memory, etc.) or some combination of the two. This most basicconfiguration is illustrated in FIG. 1 by dashed line 106.

Additionally, device 100 may also have additionalfeatures/functionality. For example, device 100 may also includeadditional storage (removable and/or non-removable) including, but notlimited to, magnetic or optical disks or tape. Such additional storageis illustrated in FIG. 1 by removable storage 108 and non-removablestorage 110. Computer storage media includes volatile and nonvolatile,removable and non-removable media implemented in any method ortechnology for storage of information such as computer readableinstructions, data structures, program modules or other data. Memory104, removable storage 108 and non-removable storage 110 are allexamples of computer storage media. Computer storage media includes, butis not limited to, RAM, ROM, EEPROM, flash memory or other memorytechnology, CD-ROM, digital versatile disks (DVD) or other opticalstorage, magnetic cassettes, magnetic tape, magnetic disk storage orother magnetic storage devices, or any other medium which can be used tostore the desired information and which can accessed by device 100. Anysuch computer storage media may be part of device 100.

Computing device 100 includes one or more communication connections 114that allow computing device 100 to communicate with othercomputers/applications 115. Device 100 may also have input device(s) 112such as keyboard, mouse, pen, voice input device, touch input device,etc. Output device(s) 111 such as a display, speakers, printer, etc. mayalso be included. These devices are well known in the art and need notbe discussed at length here. In one implementation, computing device 100includes transactional memory application 200. Transactional memoryapplication 200 will be described in further detail in FIG. 2.

Turning now to FIG. 2 with continued reference to FIG. 1, atransactional memory application 200 operating on computing device 100is illustrated. Transactional memory application 200 is one of theapplication programs that reside on computing device 100. However, itwill be understood that transactional memory application 200 canalternatively or additionally be embodiment as computer-executableinstructions on one or more computers and/or in different variationsthan shown on FIG. 1. Alternatively or additionally, one or more partsof transactional memory application 200 can be part of system memory104, on other computers and/or applications 115, or other suchvariations as would occur to one in the computer software art.

Transactional memory application 200 includes program logic 204, whichis responsible for carrying out some or all of the techniques describedherein. Program logic 204 includes logic for allowing multiple paralleltransactions to be nested under a single parent transaction 206 (asdescribed below with respect to FIG. 3); logic for allowing parallelnested transactions to nest and compose properly with other closednested transactions 208 (as described below with respect to FIGS. 7-14);logic for allowing parallel nested transactions to execute withindependent logs and to transfer ownership of locks and logs to theparent with low contention 210 (as described below with respect to FIGS.4 and 5); logic for enabling parallel nested transactions for optimisticand pessimistic reads in the same transaction 212 (as described belowwith respect to FIGS. 15-17); logic for enabling parallel nestedtransactions for in-place or buffered writes 214 (as described belowwith respect to FIGS. 7, 11 and 12); logic for allowing retry to workcorrectly with parallel nested transactions 216 (as described below withrespect to FIG. 18); logic for allowing write abort compensation maps towork with parallel nested transactions 218 (as described below withrespect to FIG. 19); and other logic for operating the application 220.

Turning now to FIGS. 3-19 with continued reference to FIGS. 1-2, thestages for implementing one or more implementations of transactionalmemory application 200 are described in further detail. In someimplementations, the processes of FIGS. 3-19 are at least partiallyimplemented in the operating logic of computing device 100. FIG. 3illustrates one implementation of the stages involved in allowingmultiple parallel transactions to be nested under a single parenttransaction. The process begins at start point 240 with providing afunction or other feature that will create a set of parallel nestedtransactions that are ready to execute (stage 242). A set of parallelnested transactions are created for a given parent transaction all atonce or lazily as needed (stage 244). Special logic is used totransactionally execute, commit, rollback, re-execute, and retry aparallel nested transaction (stage 246). When a parallel nestedtransaction finishes, it must be destroyed after it and all of itsparallel siblings have completed (stage 248). The process ends at endpoint 250.

FIGS. 4 and 5 illustrate how to allocate parallel nested transactionentries in the parent log. Parallel nested transactions are able toexecute with independent logs and to transfer ownership of locks, logs,and other data to the parent with low contention after being allocatedaccording to one of the allocation techniques described in FIGS. 4 and5. Turning now to FIG. 4, one implementation is shown for allocatingparallel nested transaction entries in the parent log when the number ofparallel nested transactions is known up front. The process begins atstart point 270 with one parallel nested transaction entry (PNTE) beingcreated for each parallel nested transaction (PNT) in the nest beforeany PNT begins execution, since the number of parallel nestedtransactions is known up front (stage 272). When needed during rollbackor commit processing, the parallel nested transaction retrieves theparallel nested transaction entry from the parent transaction withminimal synchronization. In one implementation, the pre-allocated PNTEare held in an array in the parent transaction. A PNT may obtain apointer to the next available PNTE with a simple compare and swapoperation on a next available parallel nested transaction index (stage274). A compare and swap (CAS) is an operation that atomically performsa comparison between a given value and the contents of a given memorylocation, and if they match a given new value is stored in the memorylocation. If they do not match, then no action is taken. There are manymethods to perform a CAS operation, some hardware and some software, onmany different CPU's and operating systems. The term CAS as used hereinis meant to generically cover all of these methods.

The parallel nested transaction fills in the parallel nested transactionentry in the parent log with information (stage 276). In oneimplementation, the information includes a pointer to the child's logand a pointer to the write abort compensation map for the child and/orpessimistically read object table, if applicable. The write abortcompensation map is described in further detail in FIG. 19. Thepessimistically read object table is described in more detail in FIG.15. The process ends at end point 278.

FIG. 5 illustrates one implementation of the stages involved inallocating parallel nested transaction entries in the parent log whenthe number of parallel nested transactions is not known up front. Theprocess begins at start point 290 with allocating room in the parent logfor a new parallel nested transaction entry when a parallel nestedtransaction is created (stage 292). When the parallel nested transactioncommits, a parallel nested transaction entry is created in the parentlog (stage 294). Access to the parent log is synchronized to move theparent's current log point to acquire access to the next available spacefor a parallel nested transaction entry (stage 296). The parallel nestedtransaction fills in the parallel nested transaction entry in the parentlog with information (stage 298). In one implementation, the informationincludes a pointer to the child's log and a pointer to the write abortcompensation map for the child and/or pessimistically read object table,if applicable. The write abort compensation map is described in furtherdetail in FIG. 19. The pessimistically read object table is described inmore detail in FIG. 15. The process ends at end point 300.

FIG. 6 illustrates an example structure 320 of a transactional memoryword (TMW) of one implementation with various bits 322 being used tomark various lock statuses. In one implementation, a TMW is associatedwith each object. In one implementation, a single hardware wordrepresentation is used; but numerous other representations for TMWs arepossible that allow for locking information to be stored. In the examplestructure described herein, a TMW may indicate that the associatedobject is write-locked or is not. One bit of the TMW is dedicated tothis distinction. If a TMW is not write-locked, then it contains aversion number and a count of pessimistic readers called a version/countpair 324 (V/C pair). In this state, some number of bits of the TMWrecord the version number of the object, and remaining bits represent acount of the number of transactions that currently hold a pessimisticread lock on the object. When a transaction holds a write lock in theobject, this lock can be of two kinds (distinguished by a further bit inthe TMW). Normally, the remaining bits of a write-locked TMW contain apointer to an entry in the write log of the locking transaction 326;this write log entry (WLE) contains other information about the objectand the locking. For example, it might contain the TMW value for theobject before the write lock was obtained; in another implementation, itmight contain a “shadow copy” of the object into which uncommittedmodifications are made. In the other state, a write-locked TMW containsa versioned write lock (VWL). Here, the remaining bits of the TMW(called VWL [V/C] 328) represent the version number of the object whileit is still write locked and a count of pessimistic readers, similar toa V/C pair.

Before moving on to more detailed discussions on how versioned writelocks are used, let's first explore an example that will help illustratethe need for versioned write locks (VWL's). Suppose there is an in-placeSTM system, and a top-level transaction Tx1 that acquires a write lockon object O1. The TMW for O1 is set to WLE1, which is a write log entryin Tx1's log representing Tx1's write lock on O1. Now, suppose two PNT'sare introduced, Tx2 and Tx3, with Tx1 as the parallel parent. Tx2 andTx3 are siblings in the parallel nest, and are executed concurrently.Tx2 and Tx3 may access data locked by Tx1, but they must be isolatedfrom each other. Therefore, while Tx2 and Tx3 may both access O1, theymust not be allowed to do so concurrently. Now, suppose Tx3 wishes toread from O1. It performs an optimistic read operation, creating anoptimistic read log entry in its log recording the value of O1's TMW asWLE1. Next, assume that Tx2 writes to O1. Tx2 will acquire a write lockon O1, and set the TMW for O1 to WLE2, which is a write log entry inTx2's log. WLE2 records that WLE1 is the previous value of O1's TMW. Tx2may now write to fields of O1, and does so with in-place writes. As Tx3continues to execute, it reads fields of O1 that contain uncommittedwrites from Tx2. Tx3 is by definition doomed and should rollback.However, if Tx2 rolls back for any reason before Tx3 attempts to commit,then it must release its write lock on O1. To do this, Tx2 wouldnormally set O1's TMW back to WLE1. But now when Tx3 attempts to commit,it will see that O1's TMW contains the same value that it did when Tx3first read O1. In this situation, Tx3 will appear valid, and will failto recognize that it read uncommitted writes from Tx2. Therefore, whenTx2 rolls back it must set O1's TMW to some value other than WLE1, andit must do this in a way that ensures that other transactions in thesystem (PNT siblings or other top-level transactions) recognize that O1is still write locked by Tx1. This is accomplished by setting O1's TMWto a versioned write lock (VWL) and making an entry in the globalversioned write lock map (VWLM) that indicates Tx1 holds a write lock onO1. The details and use of VWL's and the VWLM are described below. Thisexample illustrates one case where a VWL is necessary. However, it willbe appreciated by one of ordinary skill in the art that there arenumerous scenarios where a VWL can be used, as will become apparent asthe processes for lock acquisition and release are described in detailin the rest of this section.

FIG. 7 illustrates one implementation of the stages involved in usingversioned write locks to ensure parallel nested transactions nest andcompose properly. The process begins at start point 340 with providing atransactional memory word which can be one of a version/count pair, awrite log entry (WLE), or a versioned write lock (VWL) (stage 342). Whena transactional memory word (TMW) changes to a versioned write lock, anentry is made in a global versioned write lock map (VWLM), which isindexed by the object address, referring to the old write log entrypointer that the versioned write lock replaced (stage 344). When theversioned write lock is seen, the global versioned write lock map isconsulted to translate it to a write log entry pointer and normalprocessing occurs on the write log entry pointer (stage 346). Versionnumbers in the transactional memory word always increment during commitor abort processing, whether held in a V/C pair or a versioned writelock (stage 348). An entry is made in the global versioned write lockmap just before the versioned write lock is placed in the transactionalmemory word, and the entry remains so long as the transactional memoryword contains the versioned write lock (stage 350). At any point intime, the transactions have to acquire a global lock to access theglobal versioned write lock map (stage 352). The process ends at endpoint 354.

FIGS. 8 and 9 illustrate one implementation of the stages involved inproperly handling a nested transaction acquiring a write lock. A newwrite lock is a write lock acquired for the first time within a nest oftransactions, and a duplicate write lock is a write lock which a nestedtransaction may acquire on an object that an ancestor transactioncurrently has write locked. The process begins at start point 370 with anested transaction reading a transactional memory word (stage 372). Ifthe transactional memory word is not a V/C pair (decision point 374),then the process continues with FIG. 9, stage 404, described in the nextsection. If the transactional memory word is a V/C pair with C (count ofpessimistic readers) greater than zero (decision point 375), then theprocess of FIG. 16 is performed to handle pessimistic read to write lockupgrade. If the transactional memory word is a V/C pair with C equal tozero (decision point 375), this indicates that the transactional memoryword is not pessimistically locked for read or write by any transaction,therefore allowing acquisition of a new write lock. To do so, the systemforms a new write log entry to record the version number and appends thewrite log entry to the transaction's write log (stage 376). A compareand swap is then performed to switch the transactional memory word valuefrom a V/C pair to a write log entry pointer (e.g. WLE*) (stage 378). Ifthe compare and swap is successful (decision point 380), then the lockwas obtained successfully (stage 382). If the compare and swap is notsuccessful (decision point 380), then there is a conflict (stage 384)and the lock was not obtained successfully. The process ends at endpoint 386.

Continuing with FIG. 9, if the TMW is not a V/C pair (decision point 374on FIG. 8), and if the transactional memory word is not a versionedwrite lock (decision point 404), then it is a WLE* to WLE_(A) and theprocess continues at stage 408. If the transactional memory word is aversioned write lock (decision point 404), then the global version writelock map is used to retrieve the underlying WLE* to WLE_(A) (stage 406).If WLE_(A) is not owned by any ancestor of the transaction (decisionpoint 408), then there is a conflict (stage 410) and the process ends atend point 420. If WLE_(A) is owned by an ancestor (decision point 408),then a new WLE_(B) is formed to record WLE_(A) and WLE_(B) is appendedto the transaction's write log(stage 412). A compare and swap is thenperformed to switch the transactional memory word value from WLE_(A)* toWLE_(B)* (stage 414). If the compare and swap was successful (decisionpoint 416), then the lock was successfully obtained (stage 418). If thecompare and swap was not successful (decision point 416), then there isa conflict (stage 410) and the lock was not successfully obtained. Theprocess ends at end point 420.

FIG. 10 illustrates one implementation of the stages involved inproperly handling a nested transaction releasing any kind of write lockfor commit. The process begins at start point 440 with WLE_(X) beingencountered in a transaction write log during commit processing (stage442). Ownership of the write lock, and WLE_(X), is simply transferred tothe immediate parent transaction (stage 444). By transferring ownershipfrom a parallel nested transaction to the parent transaction on commit,other siblings now find that they can acquire the write lock forthemselves now. Likewise, the act of acquiring a duplicate write lockprevents siblings from being able to acquire the write lock forthemselves. The process ends at end point 446.

FIG. 11 illustrates one implementation of the stages involved inproperly handling a nested transaction releasing a new write lock forrollback. The process begins at start point 460 with WLE_(X) beingencountered in a transaction write log during rollback processing (stage462). The system checks to see what type of write lock WLE_(X)represents (stage 464). If WLE_(X) does not represent a new write lock(decision point 466), then logic for releasing a duplicate write lock isperformed (stage 471), as described in FIG. 12 for one implementation.If WLE_(X) represents a new write lock (decision point 466), then theprevious version number stored in WLE_(X) is retrieved (stage 468). Thelocation of the transactional memory word is retrieved from WLE_(X) andthe write lock is released using a normal store operation to change thetransactional memory word value to a V/C pair appropriate for the typeof system (in-place or buffered) (stage 470). In one implementation of abuffered system, the transactional memory word value is changed back torepresent the original version number. In one implementation of anin-place system, then the transactional memory word value is changed torepresent the original version number plus one. The process ends at endpoint 472.

FIG. 12 illustrates one implementation of the stages involved inproperly handling a nested transaction releasing a duplicate write lockfor rollback. In one implementation, this process is used only forsystems that increment an object's version number on rollback, i.e., anin-place system. In some implementations of buffered systems, theversion number is not increased during rollback. In these systems, theprocess for releasing a new write lock (FIG. 11) may be used to releasea duplicate write lock. The process begins at start point 490 withWLE_(X) being encountered in a transaction write log during rollbackprocessing (stage 492). The system checks to see what type of write lockWLE_(X) represents (stage 494). If the lock is not a duplicate writelock (decision point 496), then logic for releasing a new write lock isperformed (stage 511), as described in FIG. 11 for one implementation.If the lock is a duplicate write lock, then a global lock is acquiredthat is used to synchronize access to the global versioned write lockmap (stage 498). The original write log entry pointer WLE_(Y)* and thelocation of the transactional memory word are retrieved from WLE_(X)(stage 500). A new global versioned write lock map entry is formed forthe object, referring to WLE_(Y) (stage 502). The global lock that isused to synchronize access to the global versioned write lock map isthen released (stage 504). The original version number is retrieved fromWLE_(X) (stage 506), and a new versioned write lock value is formed asthe original version number+1 (stage 508). The write lock is releasedusing a normal store operation to change the transactional memory wordvalue from WLE_(X)* to the new versioned write lock (stage 510). Theprocess ends at end point 512.

FIG. 13 is a process flow diagram for one implementation of the systemof FIG. 1 that illustrates performing optimistic read lock acquisition.The process begins at start point 530 with the object's current TMWvalue being read when a transaction performs an optimistic read on anobject (stage 532). An optimistic read log entry is created (stage 534)and populated with the current TMW value and the location of the TMW(stage 536). The read log entry is appended to the transaction's readlog (stage 538). In one implementation, this process is the same for alltypes of transactions: top-level, simple nested, or parallel nestedtransactions. The process ends at end point 540.

FIGS. 14A-14D are process flow diagrams for one implementation of thesystem of FIG. 1 that illustrates performing optimistic read validation.The process begins at start point 560 with considering each optimisticread log entry in the transaction's read log when attempting to commit atransaction or otherwise determine if the transaction is valid (stage562). The original value of the TMW is retrieved from the read log entry(stage 564), and the current value of the TMW is read (stage 566). Notethat in each case, if the system is using write abort compensation maps(WACM) (described in FIG. 19) then the current aggregate WACM (formedduring validation or commit processing) is consulted whenever there is adifference in two version numbers. If the original TMW is a V/C pair(decision point 568), then the process described in FIG. 14 B isperformed. If the original TMW is a WLE* (decision point 570), then theprocess described in FIG. 14 C is performed. If the original TMW isneither a V/C pair nor a WLE*, then the original TMW is a VWL (stage572), and the process of FIG. 14D is performed. Let's look at each ofthese scenarios in further detail.

FIG. 14 B covers more details on an exemplary process performed duringoptimistic read validation in one implementation when the original TMWis a V/C pair (decision point 568). If the current TMW is a V/C pair(decision point 590), and the version numbers in the original TMW andthe current TMW match (decision point 592), then the transaction isvalid (stage 594). If the current TMW is a V/C pair (decision point590), and the version numbers in the original TMW and the current TMW donot match (decision point 592), then the transaction is invalid (596).

If the current TMW is instead a WLE* (decision point 598), and if theWLE is owned by the transaction, and the saved version number in the WLEmatches the old TMW (decision point 600), then the transaction is valid(stage 594). If the current TMW is not a V/C pair (decision point 590),and the current TMW is not a WLE* (decision point 598), then the TMW isa VWL (stage 602). A synchronized lookup is made in the VWLM using theaddress of the locked object. If there is no VWLM entry (decision point604), then the transaction is invalid (stage 596). If there is an entry(decision point 604), then the VWLM entry is used to retrieve the WLE*that the VWL replaced. If the WLE is owned by the transaction, and thesaved version number in the WLE matches the old TMW (decision point606), then the transaction is valid (stage 594). Otherwise, thetransaction is invalid (stage 596). The process ends at end point 608.

Turning now to FIG. 14 C, an exemplary process is shown that isperformed during optimistic read validation in one implementation whenthe original TMW is a WLE*. If the current TMW is a V/C pair (decisionpoint 620), then the current transaction is not valid (stage 630). Ifthe current TMW is not a V/C pair (decision point 620), but is instead aWLE* (decision point 624), then the system checks to see if the originaland current TMW match, and if the WLE is owned by the currenttransaction or any ancestor (decision point 626). If both of thesecriteria are met, then the transaction is valid (stage 628). Otherwise,the transaction is invalid (stage 630).

If the current TMW is not a V/C pair (decision point 620), and is not aWLE* (decision point 624), then the current TMW is a VWL (stage 632). Ifthe WLE from the original TMW is owned by this transaction or anyancestor, and if the version number saved in the WLE matches the versionnumber in the VWL (decision point 634), then the transaction is valid(stage 628). Otherwise, the transaction is not valid (stage 630). Theprocess ends at end point 636.

Turning now to FIG. 14 D, an exemplary process is shown that isperformed during optimistic read validation in one implementation whenthe original TMW is a VWL. If the current TMW is a V/C pair (decisionpoint 650), then the transaction is not valid due to a conflict (stage660). If the current TMW is not a V/C pair (decision point 650), butinstead is a WLE* (decision point 654), then the system checks to see ifthe WLE is owned by the current transaction or any ancestor, and if theversion number stored in the WLE matches the version in the VWL(decision point 656). If both of these criteria are met, then thetransaction is valid (stage 658). Otherwise, the transaction is notvalid (stage 660).

If the current TMW is not a V/C pair (decision point 650), and thecurrent TMW is not a WLE* (decision point 654), then the current TMW isa VWL (stage 662). A lookup is performed in the VWLM to translate theVWL to a WLE* (stage 664). If an entry is not found (decision point666), then the transaction is not valid (stage 660). Otherwise, if anentry is found (decision point 666), then the system checks to see ifthe version numbers of the original and current VWL match, and if theWLE found in the VWLM for the TMW is owned by this transaction or anyancestor (decision point 668). If both criteria are met, then thetransaction is valid (stage 658). Otherwise, the transaction is invalid(stage 660). The process ends at end point 670.

Correct operation of pessimistic reads in a system with simple closednested transactions requires the use of a duplication detection datastructure called a Pessimistically Read Object Table (PROT). Eachtop-level transaction creates a PROT either at transaction start, orlazily at the first pessimistic read operation. When the transaction, orany descendent, attempts to acquire a pessimistic read lock on anobject, it consults the PROT to determine if a pessimistic read lock hasalready been acquired. If the object is in the PROT, then that object isalready read locked by the nest of transactions. If the object is not inthe PROT, and if the object is not currently write locked by anothertransaction, then a pessimistic read lock is acquired using a CASoperation to increment C (the count of pessimistic readers) in the V/Cpair stored in the object's TMW. If the CAS operation is successful,then an entry is made in the PROT to record the fact that the nest nowhas the object locked for pessimistic reads. When the pessimistic readlock is released, either during top-level commit or rollback, C isdecremented (with a CAS again), and the PROT entry is removed. Let's nowlook at how PROTs are used with parallel nested transactions.

FIG. 15 illustrates one implementation of the stages involved inensuring correct operation of pessimistic reads with parallel nestedtransactions. The process begins at start point 690 with a parallelnested transaction creating a pessimistic duplication detection datastructure (called a PROT, as described previously) duringinitialization, or lazily during acquisition of the first pessimisticread lock by the parallel nested transaction (stage 692). This datastructure is used by the system when a transaction attempts to upgrade apessimistic read lock to a write lock. An entry is made into the PROTfor the first pessimistic read of an object made by the parallel nestedtransaction or any sequential child (stage 694). On commit, newpessimistic read locks are passed to the immediate parent, and anappropriate parent PROT entry is made (stage 696). Duplicate read locksare released to allow siblings to gain write access after a parallelchild commits (stage 698). The system then destroys log entriesassociated with duplicate pessimistic read locks (stage 700). Aftercommit, a child's PROT can be destroyed with the rest of the transaction(stage 702). The process ends at end point 704.

FIG. 16 illustrates one implementation of the stages involved inupgrading pessimistic read locks to write locks. The process begins atstart point 720 with discovering that a write lock that is desired foran object is already opened for pessimistic read(s) (stage 722). Thechild's PROT is queried (stage 724) to look at a current count ofreaders and decide if the current transaction can account for all thosereads (stage 726). If the result accounts for all of the pessimisticreaders, then the child may attempt to upgrade to write lock as usual(stage 728). If there are still unaccounted for readers, then the childmust query all ancestor PROT's to determine if it may upgrade to a writelock (stage 730). If any ancestor is a parallel parent then we mustconsider the parallel parent's PROT as well as the PROT of any parallelsiblings that have already committed. These sibling PROT's are held inthe parallel parent's log via PNTE's. Proper synchronization is requiredto ensure that there is race-free access to these sibling PROT's. Thissynchronization is accomplished by ensuring that a PNT does not accessits PROT after placing it in the associated PNTE. If the ancestors andany committed parallel siblings make up for the extra pessimisticreaders, then the child may attempt to upgrade to a write lock as usual(stage 732). In one implementation, the upgrade is accomplished asfollows. A new write log entry is formed and added to the PNT's log. Thecurrent value of the TMW is placed in the WLE for use during rollbackand commit processing. If the current value of the TMW is a VWL, thenthe VWLM is used, with proper synchronization, to translate the VWL to aWLE* first. A CAS is used to acquire the write lock. If the CAS issuccessful, then the upgrade worked, otherwise there is conflict. Theprocess ends at end point 734.

Now it is time to describe the exemplary process shown in FIGS. 17A-17Dfor performing pessimistic read lock acquisition when in a parallelnested transaction. The process begins with 17A with reading the currentTMW (stage 752). If the TMW is a V/C pair (decision point 754), then theprocess described in FIG. 17 B is performed. If the TMW is a WLE*(decision point 756), then the process described in FIG. 17 C isperformed. If the TMW is not a V/C pair (decision point 754), and is nota WLE* (decision point 756), then the TMW is a VWL (stage 758), and theprocess described in FIG. 17 D is performed. Each of these scenarioswill now be looked at in further detail.

Turning now to FIG. 17 B, if the TMW is a V/C pair, then thetransaction's PROT is consulted to determine if it already holds apessimistic read lock on the object (decision point 769). If there is aPROT entry, then the process ends at end point 776. If there is no PROTentry (decision point 769), then a compare and swap (CAS) is used toincrement C, which is the count of pessimistic readers (stage 770). Ifthe CAS is successful (decision point 772), then the lock is acquiredand a PROT entry is made (stage 774). If the CAS fails (decision point772), then try again, which is illustrated by referring to FIG. 17 A,stage 752.

As shown in FIG. 17 C, if the TMW is a WLE*, and the WLE is owned by thecurrent transaction or any ancestor (decision point 790), then thesystem determines if the owning transaction is below or above theparallel parent (decision point 792). If the owning transaction is belowthe parallel parent (decision point 792), then the pessimistic readsucceeds (stage 794). If the owning transaction is above the parallelparent (decision point 792), then the system switches the TMW to a VWLto coordinate with siblings. To do so, a VWLM entry is made to recordthe WLE* (stage 796), with proper synchronization, and a VWL is formedfrom the version number stored in the VWL, and with C (the count ofpessimistic readers) set to 1 (stage 798). The new VWL is set in the TMWwith a CAS (stage 800). If the CAS is successful, a PROT entry is madeand the pessimistic read lock is acquired (stage 806). If the CAS is notsuccessful, then the VWLM entry is removed (stage 804), and then we tryagain by returning to FIG. 17 A, stage 752. The process ends at endpoint 810.

As shown in FIG. 17D, if the TMW is a VWL, then the VWL is translated toa WLE* via the VWLM (stage 820), with proper synchronization. If the WLEis owned by this transaction or any ancestor (decision point 822), thena CAS is used to increment C (count of pessimistic readers) in the VWLin the TMW (stage 824). If the CAS is successful (decision point 826),then a PROT entry is made and the lock is acquired. If the CAS fails,then we try again by continuing with FIG. 17A, stage 752. If the WLE isnot owned by this transaction or any ancestor (decision point 822), thenthere is a conflict (stage 830). The process ends at end point 832.

FIG. 18 illustrates one implementation of the stages involved inallowing retry to work correctly with parallel nested transactions.Before delving into the details of FIG. 18 and the discussion ofallowing retry to work with parallel nested transactions, it is firstnecessary to provide some background information on retry operations ofone implementation. A retry operation allows basic communication betweentransactions. When a transaction executes a retry operation, its effectsare rolled back and execution is suspended until something that thetransaction read changes. When a change is detected, the transaction isre-executed. Retry operations can be used for some very common datastructures, like blocking queues. For example, a transaction could checkto see if a queue is empty and then retry if it is empty, or remove anelement if the queue is not empty. The transaction will block while thequeue remains unchanged and re-execute when the state of the queuechanges, which gives the transaction another opportunity to complete.

In one implementation, when a transaction executes a retry operation,the system registers for waits on every read in the read set of theretrying transaction. The retrying transaction waits for notificationthat something in the read set has changed. A wait notification isinitiated from a particular transaction releasing a write lock. Atransaction knows if notification is required in one of two ways. In thefirst way, if the transactional memory word contained a waiters bitduring write lock acquisition, then during release the transactionalmemory word is looked up in an object waiters map, and each waitingtransaction is signaled. In the second way, if a writing transactionfinds a global count of waiting transactions to be greater than zeroafter releasing all write locks, it will use a transaction waiters mapto determine which transactions, if any, were waiting on locationswritten to and need to be signaled. In each case, the write lock isreleased using a normal store operation.

In another implementation, a progressive retry operation is started witha rollback of just the retrying nested transaction and waiting on itsread set. Alter waiting for some particular time or for some othercondition to be met, a backoff process is performed to rollback animmediate parent of the retrying transaction, increasing the size of theoriginal wait set. The backoff process is repeated until rollback of atop-most parent, adding an additional wait for each next parent. Theaggregate wait set is associated with the top-most parent and anynotification will result in re-execution of the top-most parent.

Returning now to FIG. 18, an explanation of how retry can be used withparallel nested transactions will now be discussed. The process beginsat start point 850 when a parallel nested transaction or any sequentialdescendant executes a retry, the system registers the transaction's readset for retry as usual (stage 852). The system includes heuristics todetermine how long a retrying transaction should wait before expandingthe retry operation to include some subset of the transaction'sancestors (stage 854). These heuristics are adjusted for parallel nestedtransactions and allow an extended wait time so long as other parallelsiblings are active. If that extended wait time is exceeded, then theretry operation is expanded to include the parent (stage 856). The stateof the siblings is considered before propagating further (stage 858).When a decision is made to propagate a retry past a parallel nestedtransaction parent, all children are doomed, and then must finish beforea retrying parent is rolled back (stage 860). When a decision is made topropagate a retry past the parent, the parallel nested transaction'sread set remains registered and becomes part of a parent read set (stage862). Note that the read sets of any parallel siblings that have alreadycommitted into the parent are made part of the wait set along with theparent's read set. Any parallel siblings that were aborted by thedecision to propagate the retry to the parent do not contribute to thewait set. The process ends at end point 864.

FIG. 19 illustrates one implementation of the stages involved in usingwrite abort compensation maps with parallel nested transactions. Beforegetting into the details of FIG. 19 and how write abort compensationmaps can be used with parallel nested transactions, it is necessary toprovide some background information regarding write abort compensationmaps of one implementation.

Write abort compensation maps can be used to detect falsely doomedparent transactions of nested children in transactional memory systemsthat used in-place writes. A write abort compensation map (or otherstorage mechanism) tracks the release count for each lock released foreach nested transaction that rolls back. The number of times the nestedtransactions release a write lock is recorded in their respective writeabort compensation map. The release counts can be used during avalidation of a parent transaction to determine if an apparently invalidoptimistic read is really valid.

In one implementation, while processing a parent transaction log, anywrite abort compensation maps seen for nested child transactions arecombined into an aggregated write abort compensation map in the parent.If the optimistic read failed to validate due to a version numbermismatch, then the aggregated write abort compensation map is consultedto retrieve a particular variable's write lock release count for thenested child transactions. If a difference in version numbers exactlymatches the write lock release count for the nested child transactions,then the optimistic read is valid.

Returning now to FIG. 19, let's look at how write abort compensationmaps can be used with parallel nested transactions. The process beginsat start point 880 with creating a write abort compensation map (WACM)when releasing new write locks during rollback of a parallel nestedtransaction (stage 882). When a nested transaction rolls back, itcreates a WACM entry for each new write lock released (stage 884). TheWACM is logically placed before the parallel nested transaction's logwhen ownership of the log is transferred to the parent during commit(stage 886). These PNT WACM's will be used during parent rollback justlike any other nested transaction's WACM, as described above, and willensure correct validation of parent optimistic reads (stage 888). Theprocess ends at end point 890.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims. All equivalents, changes, andmodifications that come within the spirit of the implementations asdescribed herein and/or by the following claims are desired to beprotected.

For example, a person of ordinary skill in the computer software artwill recognize that the examples discussed herein could be organizeddifferently on one or more computers to include fewer or additionaloptions or features than as portrayed in the examples.

1. A computer-readable medium having computer-executable instructionsfor causing a computer to perform a method comprising: creating, withina shared software transactional memory, a plurality of closed nestedtransactions for a single parent transaction, wherein the closed nestedtransactions and the single parent transactions are software supportedtransaction; executing, within the shared software transactional memory,the closed nested transactions concurrently as parallel nestedtransactions, including; allowing, with software, effects of one of theparallel nested transactions to roll back if the one parallel nestedtransaction is discovered to be in conflict with some other concurrenttransactions; and hiding effects of the parallel nested transactionsfrom other transactions outside the parent transaction until the parenttransaction commits in the shared software transactional memory.
 2. Thecomputer-readable medium of claim 1, wherein the parallel nestedtransactions are created all at once.
 3. The computer-readable medium ofclaim 2, wherein a parallel nested transaction entry is created in a logof the single parent transaction at create time for each of the parallelnested transactions.
 4. The computer-readable medium of claim 3, whereina respective parallel nested transaction entry is retrieved from the logof the single parent transaction by a respective one of the parallelnested transactions using a compare and swap operation on a nextavailable parallel nested transaction index.
 5. The computer-readablemedium of claim 1, wherein when a lock entry is encountered in a log ofa given parallel nested transaction during commit processing, ownershipof a lock and the log entry is transferred to the single parenttransaction.
 6. The computer-readable medium of claim 1, wherein theparallel nested transactions are created as needed.
 7. Thecomputer-readable medium of claim 6, wherein as each respective parallelnested transaction is created, space is allocated in a log of the singleparent transaction for a respective parallel nested transaction entry.8. The computer-readable medium of claim 7, wherein when each respectiveparallel nested transaction commits, the respective parallel nestedtransaction entry is created in the log of the single parenttransaction.
 9. The computer-readable medium of claim 7, wherein the logof the single parent transaction is synchronized for accesses amongsibling parallel nested transactions.
 10. A method for supportingparallel nested transactions in a software transactional memory system,the method comprising: creating, within a shared software transactionalmemory, a plurality of closed nested transactions for a single parenttransaction; executing the plurality of closed nested transactions ofthe parent transaction in parallel in a shared software transactionalmemory, wherein the closed nested transactions and the single parenttransactions are software supported transaction; allowing, withsoftware, effects of one of the parallel nested transactions to rollback if the one parallel nested transaction is discovered to be inconflict with some other concurrent transactions; and hiding effects ofthe parallel nested transactions from other transactions outside theparent transaction until the parent transaction commits in the sharedsoftware transactional memory.
 11. The method of claim 10, furthercomprising: hiding effects of a given parallel nested transaction fromother parallel nested transactions of the parent transaction until thegiven parallel nested transaction is committed into the parenttransaction in the shared software transactional memory.
 12. The methodof claim 10, wherein the parallel nested transactions use in-placewrites.
 13. The method of claim 10, wherein the parallel nestedtransactions use buffered writes.
 14. The method of claim 10, whereinthe parallel nested transactions use optimistic reads.
 15. The method ofclaim 10, wherein the parallel nested transactions use pessimisticreads.
 16. A computer-readable medium having computer-executableinstructions for causing a computer to perform the steps recited inclaim 10.